mpu6000: add & check for the device_type on driver startup

Fixes the following case: a board that has several bus_options enabled,
and wants to start a specific device (eg mpu6000 -T 20608 start). In that
case the given device_type is never actually checked, and since the start
routine iterates over all bus_options, several can match, and thus start
multiple instances of the driver (note that the whoami check is performed
against the value provided with -T and thus the check will pass).

This happens on Pixracer (PX4_SPIDEV_MPU and PX4_SPIDEV_ICM_20608 are both
set).